The basic problem involved in the roasting of cocoa beans or cocoa kernel fragments lies in the relatively large dimensions of the particles of roasting material and their widely different size distribution. This results in large temperature gradients between the surfaces of the particles of roasting material and their centers. Overroasting and/or underroasting thereby become virtually unavoidable.
This problem has been recognized for more than 50 years and has led to the notion of crushing raw cocoa beans into a pulpy or liquid cocoa paste, spreading the latter in a thin layer on smooth, moving surfaces heated to roasting temperature, and roasting the paste by heating the same to approximately 100.degree. to 150.degree. C.
There has also been previously described in U.S. Pat. No. 3,955,489 (Goerling et al.) a process which provides for the continuous preparation of roasted cocoa paste from a thin liquid paste made from dried, shelled and crushed cocoa beans. The paste is conveyed continuously in a layer having a thickness of 1 to 2 mm. within a period of time having a duration of 1 to 5 minutes over a surface heated to a maximum of 150.degree. C. A gas is directed over the thin layer in countercurrent thereto and the cocoa paste leaving the heating surface is cooled immediately to 80.degree. C.
The above process is intended to avoid the defects of known processes which have found no acceptance in practice and to achieve various advantages by using a thin-layer evaporator. However, comparative tests conducted with this process have shown that even such a process does not lead to the desired achievement of optimal aroma formation with an economically justifiable expenditure. This may be the reason why this process, also, as far as is known, has likewise found no acceptance in cocoa processing practice.
In terms of equipment, an obstacle standing in the way of the practical realization of this known process is that a disproportionately large area is required if a layer having a thickness of approximately 0.5 mm. and a conventional throughput of 1,000 kg./h. is adopted. With a necessary duration of the paste in the roasting region of approximately 12.25 minutes and a layer width of 1 m., the result is a belt length of 340 m. which is difficult to put into practice and demands a corresponding plurality of thin-layer evaporators.
If there were to be used for the roasting process, known thin-layer evaporators in which a 4-blade rotor, engaging in the layer of paste and having a diameter of 300 mm. and a length of 2.2 m., rotates in a vertical, tubular housing heated from outside, then 14 such evaporators would be required for a throughput of 1,000 kg./h., allowing for a corresponding layer volume.
Furthermore, if considered for roasting by this process, thin-layer evaporators have, on an average, a clearance of around 1.5 mm. between the scrapers and the wall. Narrower production tolerances are economically impractical for equipment of the required size, apart from the fact that a sufficiently large clearance must remain to insure that a thin layer can form again behind the rotor blades. The layer picked up by the rotor may partly stay for an uncontrollably long time on the heated wall and is therefore necessarily overroasted.
In addition, the paste develops a strong CO.sub.2 formation in the form of very fine bubbles and swells to approximately double the volume. However, since the boundary layer towards the wall cannot be scraped, strong thermal insulation occurs which considerably worsens the transfer of heat from the wall to the paste. This is due to the fact that the coefficient of thermal conductivity of the cocoa paste is 0.3 kcal/m.h..degree.C. whereas the coefficient of thermal conductivity of CO.sub.2 gas is 0.02 kcal/m.h..degree.C.
It is clear from this that economically appropriate roasting is not possible with known thin-layer processes. The results obtained in comparative testing under optimal thin-layer conditions also show that the cocoa paste thus prepared is not refined thereby to its final state for the purpose of finishing (tumbling in a conche). This is because the paste is not yet sufficiently dehydrated, deacidified and degassed. This means that the chocolate paste prepared therefrom will have to be tumbled in a conche for a relatively long time, say approximately 24 hours, or else that the cocoa paste roasted by thin-layer processes will have to undergo further expensive thin-layer treatment for the purpose of degassing, dehydration, and deacidification, in order to reduce energy-consuming and time-consuming tumbling in a conche to half to one third of the time which would otherwise be required.
Regarding the question of aroma formation during roasting, the following must be borne in mind with respect to the so-called preliminary aroma steps: the formation of cocoa aroma during roasting of dry raw cocoa presupposes that its preliminary steps are present and can react with one another. Amino acids and reducing sugars which are to be considered as preliminary steps arise due to postmortal, enzymatic hydrolysis in the water-containing seed in the course of fermentation. Since fermentation never takes place under optimal conditions and even takes place, as a rule, under relatively poor conditions, the preliminary aroma steps are distributed throughout the individual beans in a widely different concentration. Crushing the dried and shelled cocoa beans into a cocoa paste does not itself provide any substantial improvement in this respect since here, also, the preliminary aroma steps are of necessity distributed through the solid particles of cocoa in widely different concentrations. Consequently, a homogenous distribution of preliminary aroma steps cannot be achieved solely by a fine distribution of the solid particles of cocoa in the cocoa butter of the cocoa paste.